Add disconnected connection type for Update Engine

Add a network disconnected connection type to the Update Engine
connection type histogram.

BUG=chromium:660283

Change-Id: Icc93ec2f49073566dcb49d6abcd70bc535aca176
Reviewed-on: https://chromium-review.googlesource.com/c/1235282Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>
Commit-Queue: Colin Howes <chowes@google.com>
Cr-Commit-Position: refs/heads/master@{#608884}

tools/metrics/histograms/enums.xml

@@ -50643,6 +50643,7 @@ Full version information for the fingerprint enum values:
   <int value="5" label="Cellular"/>
   <int value="6" label="Tethered (Ethernet)"/>
   <int value="7" label="Tethered (Wifi)"/>
+  <int value="8" label="Disconnected"/>
 </enum>
 
 <enum name="UpdateEngineDownloadErrorCode">